This invention is related to pumps and pumping systems. More specifically, this invention is directed to the dynamic measurement of vapor pressure in pumping systems.
In the art of pumps and pumping systems, Net Positive Suction Head Available (NPSHa) pressure is a well-known operating characteristic that is necessary for maintaining proper operation. Maintaining system NPSHa higher than Net Positive Suction Required (NPSHr) by the pump is critical as it insures a smooth flow of the fluid entering and leaving a pump. NPSHa conventionally is determined by measuring or estimating known system operating parameters and applying these values in the well-known formula:
NPSHa=(Psxe2x88x92Pv)/SG+xcex94Zs+hvsxe2x80x83xe2x80x83[1]
where Ps is pump suction absolute pressure, in feet;
Pv is pumpage vapor pressure, in feet;
SG is pumpage Specific Gravity;
xcex94Zs is the difference in suction gauge height to pump suction input data, in feet; which is positive if gage is above pump datum and negative if gage is below pump datum; and hvs is suction head velocity, in feet.
Conventionally, the measurement of each of the operating parameters is not or cannot be measured as the pump is in operation. Generally, when the parameters are not measured, their values are estimated based on known characteristics, such as pump size, type of fluid, fluid viscosity, temperature, fluid flow rate, etc. In such case, the NPSHa is statically determined based on the information available when the pump system is set up.
However, conditions within the pumping system can change which can induce significant changes in the operating conditions and alter the actual NPSHa of the pumping system. For example, cavitation is a well-known problem in pumping systems that can alter the NPSHa. Cavitation occurs when air, in the form of bubbles, is released from the pumped fluid and explode against the high speed pump impeller blades. The exploding air bubbles cause ever increasing damage to the impeller blades and the damaged impeller blades are detrimental to the smooth flow of the pumped fluid. In such cases, additional air bubbles are released from the pump and the cavitation level increases. As the cavitation level increases, the vapor pressure within the pump increases and, from Equation 1 above, the NPSHa pressure decreases.
However, vapor pressure is not a measurable parameter in the conventional pumping systems. Thus, a change in vapor pressure can occur and the change in actual NPSHa can cause significant damage to the pumping system or the system that is using the pumped fluid. Hence, there is a need in the art to dynamically measure vapor pressure to be able to determine NPSHa pressure when the pump is in operation.
A method and device for dynamically determining a fluid vapor pressure passing through a pumping system is disclosed. The device measures the fluid vapor pressure by diverting a portion of the fluid from the pumping system into a vapor pressure measuring device chamber, isolating the diverted fluid from the pumping system, evacuating the chamber and measuring the vapor pressure of the isolated fluid. In another aspect of the invention, a temperature compensation device can be included in the vapor measuring device chamber to change the temperature of the chamber fluid so the fluid temperature in the chamber is substantially the same as the fluid in the pumping system.